In an electric power distribution system with alternated voltages and currents, the power factor is the ratio of real power (Watts) feeding a load to the apparent power (Volt-Amperes) absorbed thereby.
The optimum value for this ratio is unity, a value which is obtained only when the line current is sinusoidal and in phase with the line voltage, (which is sinusoidal).
This means that any current component in quadrature with the fundamental and any component at frequency other than the fundamental, cannot carry any real power to the load.
However these components contribute to total line losses and because they add to the active current actually required by the equipment, they mandate the use of heavier wiring and circuit breakers.
In the past, the main cause of low power factor was the inductive characteristic of the loads, such as electric motors and could be corrected at large extent by suitable rephasing capacitors.
With the advent of the electronic industry and the enormous increase of equipment incorporating voltage regulators having rectifiers, followed by capacitive input filters, the nature of the problem has changed.
The current drawn by these circuits is distinctly non sinusoidal with relevant armonic components.
The resulting power factor may be as low as 0,5 and, if the power involved is higher than some hundreds of watts, cost considerations alone make it imperative to search for the use of arrangements which improve the power factor.
In addition recent regulations impose that the power factor of the equipment cannot drop below predetermined limits and that the armonics contents of the drawn current be limited too.
For these reasons power factor pre-regulators have been developed for feeding post regulators with a current pulsed at high-frequency (in the order of 50-100 Khz) and sinusoidally modulated in phase with the powering voltage.
The propagation upstream of the high frequency armonic components can be easily filtered.
In addition the pre-regulators can perform as adapters to differing main voltages and provide an output at a pre-regulated voltage of predetermined value which by means of DC/DC converters is converted in one or more DC voltages regulated with very high precision.
In general such pre-regulators are structured as a conversion power stage of the "boost" type, or voltage elevator, comprising an inductor fed by a rectified voltage through a periodically closed switch.
When the switch is closed, the inductor is charged and stores magnetic energy.
When the switch is open, the inductor discharges, though a diode, on an output capacitor of the pre-regulator and transfers to it the stored magnetic energy.
An example of a pre-regulator of this kind, and of an integrated circuit which performs all the control functions of the pre-regulator is described in the document: Application Note by Claudio de Sa e Silva "power Factor Correction with the UC3854" published by the firm UNITRODE as well as in the UNITRODE technical specifications of the UC3854 component,
This component comprises overcurrent protection circuits which temporarily block its operation when the current drawn from the pre-regulator exceeds a predetermined value, as it may result from overloads or short circuits, but it does not provide any overload or failure to operate indication which could be used as an alarm signal and control signal for switching off the power supply or the whole electric equipment, power supply included, according to suitably programmed procedures.
It is clear that during the time intervals in which the pre-regulator does not operate, the power supply, if not switched off, continues to operate without complying with the power factor and distortion limits imposed by the regulations.
If this behaviour can be tolerated for short time intervals, it is unacceptable at long term and in addition to the non compliance with the regulation may be of detriment to the performances of the power supply and the equipment powered therefrom.
It may be cause of further failures and misfunctioning. The same inconvenient occur if, due to an internal failure the pre-regulator permanently stops to operate.